1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device that has a circuit configured with a thin film transistor (hereinafter referred to as the TFT). For example, the present invention relates to an optoelectronic device, such as a liquid crystal display panel, and an electronic apparatus comprising such an optoelectronic device as a component.
In the present specification, the term “semiconductor device” refers in general to such a device that exhibits its intended function by employing semiconductor characteristics. Accordingly, all of an optoelectronic device, a semiconductor circuit, and an electronic apparatus are included in the semiconductor devices.
2. Description of the Related Art
Recently, techniques for constituting a thin film transistor (TFT) by employing a semiconductor thin film (having a thickness approximately in the range from several nm to several hundreds nm) formed on a substrate that has an insulating surface has been drawing much attention. The thin film transistor has been widely used in various electronic devices such as an IC and an optoelectronic device. In particular, development of the thin film transistor as a switching element for an image display device has been urged.
A TFT employing a crystalline semiconductor film (typically a polysilicon film or the like) as its semiconductor film has been widely used more often since the crystalline semiconductor film has a larger mobility that that of an amorphous semiconductor film (typically an amorphous silicon film or the like).
However, the polysilicon TFT has an disadvantage in that impurities, defects or the like in a channel forming region tend to have significant adverse effects on TFT characteristics, especially on a threshold characteristic, although the polysilicon TFT has many advantages over the amorphous silicon TFT.
For example, a negative shift of a threshold voltage from 0 V leads to a normally-on characteristic, thereby resulting in the situation in which a normal switching operation cannot be realized.
In order to overcome these problems, it has been known to control a threshold value of a TFT by adding boron ions or the like into a channel forming region with an ion doping apparatus or an ion implantation apparatus.
In general, in an IC fabrication process, impurity ions are selectively implanted with an ion implantation apparatus. In such an ion implantation apparatus, impurity ions are accelerated by means of an electrical field and then a mass separation is performed, so that only target ions are implanted. This ion implantation apparatus can exhibit high precision, although the apparatus is very expensive and its throughput is low. The ion implantation apparatus is not suitable particularly for mass-production of an active matrix display device in which a large-size substrate has to be processed. In view of the above fact, for mass-production of the active matrix display device in which a large-size substrate has to be processed, an ion doping apparatus is typically employed since it can realize a batch process for adding impurity ions into a large-size semiconductor thin film.
In this ion doping apparatus, source material gases are flowed into a chamber and plasmarized therein by a known method, thereby ionizing the contained impurity ions to be added to a crystalline semiconductor film. Although other ions than the target ion species may be added into the film since no mass separation is performed, a satisfactory throughput can be realized.
In the above-mentioned conventional method for controlling a threshold value, it is required to set an amount of boron ions to be added into the channel forming region to an extremely small level. However, when the ion doping apparatus is employed, it has been difficult to precisely control the boron ions of a minute amount to be doped into the channel forming region.
For example, in the case where the boron ions are added into a region to become a channel forming region in a sample (semiconductor substrate) with an ion doping apparatus, boron concentration distributions as shown in FIGS. 23 to 25 can be usually obtained.
Specifically, FIGS. 23 to 25 show graphs indicating boron concentration distributions (obtained by SIMS measurements) in the case where the boron ions are added at an accelerating voltage of 80 keV in accordance with the conventional method. In each of FIGS. 23 to 25, the horizontal axis represents the depth, while the vertical axis represents the concentration.
Moreover, the impurity concentrations contained in the sample prior to the doping process were also measured. FIG. 26 shows the concentration distribution of hydrogen (H) contained in the sample prior to the doping process. Similarly, FIG. 27 shows the concentration distribution of carbon (C), while FIG. 28 shows the concentration distributions of oxygen (O) and nitrogen (N).
As shown in FIGS. 23 to 25, it has been clearly observed that nitrogen (N), oxygen (O), carbon (C), and hydrogen (H) are also added through the doping process in addition to boron. Thus, it is clear that during the doping, the component of the ambient atmosphere are also added into the sample. The present inventors have come to the idea that the thus added ambient atmospheric components may cause the TFT characteristics to be varied.
Since the doping amount has to be controlled depending on various parameters (an RF power, a frequency, a degree of vacuum, a gas concentration, or the like), these parameters are required to be always maintained in the certain ranges. However, even when these parameters were set within the certain ranges, the threshold values varied.
In accordance with the conventional ion doping method, the TFT characteristics such as the threshold characteristic may also be varied due to the ambient atmospheric components unintentionally added, thereby resulting in the fact that the TFT characteristics cannot be precisely controlled.